In many industrial processes, the monitoring and control of the temperature of the materials processed is of great importance and, for this purpose, it is necessary to measure with accuracy the value of the temperature of such materials, directly inside the machine in which the processing takes place.
A typical case of such a requirement is represented by an internal mixer for rubbers, polymers and similar materials, in which the temperature resulting from the internal friction during mixing or blending, must be controlled in order to avoid undesirable modifications to said materials.
Such temperature measurements may be carried out with electrical transducers, in particular by means of thermocouples, composed of different metals connected together at their ends; one of the connections, known as the hot junction, is then placed in contact with the material whose temperature is to be measured, and the other connection, known as the cold junction of the thermocouple, is located at a different, known temperature. Under these conditions, a voltage is produced between the two metals, which voltage may be detected and, suitably amplified, constitutes the indication of the temperature of the environment to be measured.
An apparatus of this type, however, has proved to be not very well suited for operation in industrial environments and, in particular, in such mixers with sufficient accuracy because, although they may be of sufficient sensitivity, the electrical signals supplied by the thermoelectrical effect are of extremely small value and are influenced by the perturbations created by electrical components present in the vicinity, which have values higher than that of the useful signal.
Furthermore, it must be possible to transmit the indication of temperature from the machine at which it is picked up to a control board, for example at a distance of several meters. For this purpose the signal must be amplified without, however, involving an amplification also of the perturbing signals, which would render the useful signal indistinguishable.